The present invention relates to a hydraulic control system and pertains particularly to combined automatic and manual hydraulic control means for a motor grader blade which provides sensitive precise control of the blade position during the automatic mode of operation, with provision to isolate the automatic portion of the system when in the manual mode of operation to reduce leakage and provide a more efficient control circuit.
Hydraulic control circuits for motor grader blades having both manual and automatic modes of operation are presently known as exemplified by Breitbarth et al. U.S. Pat. No. 3,229,391, and Page et al. U.S. Pat. No. 3,486,564 both of common assignment herewith. In these systems, as is best shown in the Breitbarth et al. patent, a pair of electrohydraulic valves are connected to the blade lift cylinders in parallel with a pair of manual control valves for either manual or automatic control of the blade position. When the automatic mode of operation is initiated by the operator, an electric machine attitude sensing mechanism controls the electrohydraulic valve to provide precise adjustment of the blade relative to the vehicle to maintain a level cut, grade, or slope (with one side actuated) at all times. The manual blade lift control valves are under operator control for manually positioning the blade during periods of operation when the automatic blade control is inactive. With these systems, the pump supply line and the motor lines are in constant communication with the very sensitive automatic blade control valve spool even during the manual mode of operation. In order to achieve precise control by instantaneous movement of the blade as the result of minute movement of the automatic control valve spool, leakage rate through such spool is extremely high, thus detrimentally affecting the efficiency of the manual mode of operation of the blade lift circuit. As a result, in order to achieve satisfactory speed of blade movement during the manual mode of operation, it is sometimes necessary to provide an oversize pump to compensate for the excess leakage through the automatic control valve. Another problem associated with such a system is drift of the implement caused by cylinder holding fluid leakage across the automatic blade control valve spool when in the manual mode of operation, requiring frequent correction by the operator.